Replace Poetry with PDM, Flake8/isort with Ruff

[justinmayer]

Also fixes a failing test that did not take into account the arrow Unicode specifier that was changed in #11.

[justinmayer]

@MarkoCindric-xyz: It seems there was a test that was not updated to reflect the changed Unicode specifier for the arrow character. That test was therefore failing in your pull request, but that failing test was obscured by an unrelated tooling error that I somehow did not notice when I merged the PR. In short, my fault — not yours! 😉

I updated the failing test here to reflect the changed Unicode specifier.

While the continuous integration (CI) system would normally have caught the problem, in the future you can also run the tests locally to catch these kinds of things before submitting the pull request. If you have any questions about how to do that, by all means please let me know. Cheers!